Low-level radioactive wastewater treatment in the field of nuclear industry in China always adopts a traditional three-stage treatment process, namely flocculation and sedimentation-evaporation-ion exchange. In recent years, a membrane technology is gradually applied in the field of low-level radioactive wastewater treatment and shows a tendency to replace the traditional process. The membrane technology has a wide treatment range and can sequentially remove particulate matters, colloids, organic matters and other impurities in water, and remove most of salts and radionuclides, thereby enabling the radioactivity of outlet water to achieve a relatively low level. Utilizing an inorganic adsorbent to remove nuclides is also a method for low-level radioactive wastewater treatment. The inorganic adsorbent has high selectivity for trace nuclide ions and high decontamination efficiency, can selectively adsorb the trace nuclide ions from radioactive wastewater with high salinity, and is suitable for treatment of the discontinuously produced disperse radioactive wastewater with single type of nuclides. In addition, the inorganic adsorbent has properties of good thermal stability and chemical stability and strong radiation resistance, and a saturated inorganic material has high stability in long-term geological storage and is easy to treat and dispose.
Main radioactive substances contained in the radioactive wastewater generated under operation and accident conditions of a nuclear power plant comprise long-lived fission products 134Cs/137Cs and the like with β radioactivity. A metal ion (Ti, Co, Cu, Zn, Ni, Zr and the like) stabilized ferrocyanide can efficiently and selectively adsorb Cs ions from the low-level radioactive wastewater with high salinity within the pH range of 1-13, and the distribution coefficient of the Cs ions can reach 104-106 (Nuclear Science and Engineering, 137, 206-214, 2001).
The particle of power ferrocyanide has small size, it is difficult in solid-liquid phase separation and is difficult to be directly applied to the radioactive wastewater treatment. Aiming at this problem, domestic and foreign counterparts generally adopt formed silicon dioxide as an immobilizing carrier to load the ferrocyanide (Separation and Purification Technology 16, 147-158, 1999) or combine the ferrocyanide with PAN to prepare inorganicorganic hybrid small balls (Chinese patent CN1319849A). The particle size of the adsorbent prepared according to the above methods is millimeter grade mostly, the mechanical properties of the adsorbent are improved, and the adsorbent can be used for loading a fixed bed, but the reaction efficiency and the adsorption capacity are reduced. This is because that, in the adsorption process occurring on an inorganic adsorbentsolution interface, the adsorption rate depends on two processes, namely the diffusion process from the solution to an interface layer and the internal diffusion process of the adsorbent particles, and the adsorption rate is inversely proportional to r (r is the radius of the particles) and r2 respectively, so that increasing the particle size is often at the expense of adsorption mass transfer kinetics. In addition, increasing the particles of the ferrocyanide can result in incomplete utilization of the adsorbent in an inner layer (Nuclear and Radiochemistry, 23, 108-113, 2001).
Invention Contents
The invention designs and develops a magnetic composite adsorbent with a core-shell structure with respect to the problem of difficult solid-liquid phase separation caused by direct application of ferrocyanide powder and the problems of large particle size, low adsorption capacity and poor mass transfer condition of a composite adsorbent in an existing immobilization technology, and the magnetic composite adsorbent takes a composite magnetic carrier Fe3O4 coated with a single layer SiO2 on the surface as a base to construct a micron grade magnetic core coated with ferrocyanide composite adsorbent. The adsorbent has a multi-layer structure and is characterized in that a core of the adsorbent is magnetic Fe3O4 nanoparticles, the particle size range is 10-60 nm and the specific saturation magnetization is more than 75 emu/g; the SiO2 dense single layer is coated on the surface of the magnetic core Fe3O4 to achieve the effects of stabilizing the magnetism of the material and improving the acid and alkali resistance of the material; a hydrated metal oxide MOx.H2O (M═Ti, Co, Cu, Ni, Zn and Zr) single layer is coated on the surface of Fe3O4/SiO2 to serve as a transition layer; stabilized metal ions M in the transition layer react with a potassium ferrocyanide solution to form Fe3O4/SiO2/K4−yMx[Fe(CN)6], thereby coating active components of the adsorbent on the outermost layer of the material.
According to the invention, the metal ion stabilized ferrocyanide adsorption material is coated on the surface of the Fe3O4 magnetic core to construct the micron dimension magnetic composite adsorbent, so that the particle size of the adsorbent can be greatly reduced, the specific surface area is increased and the adsorption efficiency and adsorption capacity for Cs ions are further improved. In addition, by pre-arranging the magnetic material in the adsorbent, and an external magnetic field is utilized during working, so that solid-liquid phase separation and recovery of the adsorbent can be fast realized, and the problem of difficult solid-liquid phase separation caused by reducing the size of the adsorbent is further avoided.
The technical scheme of the invention is as follows:
1. Prepare a hydrated metal oxide transition layer on the surface of Fe3O4/SiO2 
1) Prepare a hydrated titanium oxide and zirconium oxide transition layer by a sol-gel method
a) Dissolve tetrabutyl titanate in isopropanol, and control the volume ratio of the tetrabutyl titanate to the isopropanol at 0.005:1-0.05:1 to form a solution A1 for later use. Dissolve zirconium isopropoxide in isopropanol, and control the volume ratio of the zirconium isopropoxide to the isopropanol at 0.01:1-0.1:1 to form a solution A2 for later use. Mix isopropanol with ultra-pure water having resistivity of not less than 16 MΩ·cm to form a solution B for later use, wherein the volume ratio of the isopropanol to the water is 5:1-2:1.
b) Add a composite magnetic carrier Fe3O4/SiO2 coated with a dense single layer SiO2 on the surface into the solution B, perform ultrasonic dispersion for 30 min, then add concentrated ammonia water, and uniformly stir by using a polytetrafluoroethylene stirrer. Control the concentration by mass-to-volume ratio of the Fe3O4/SiO2 to the solution B within the range of 0.005-0.02 g/mL and the volume ratio of the concentrated ammonia water to the solution B within the range of 0.02:1-0.05:1.
c) Slowly drop the solution A1 into the reaction system at room temperature, control the amount of tetrabutyl titanate added into per gram of Fe3O4/SiO2 at 2.0-3.0 mL, and perform stirring reaction at room temperature for 4-6 h after dropping. Separate an obtained precipitate by using an external magnetic field, wash with anhydrous ethanol, and dry in a vacuum oven at 60-80° C. for 10-12 h to obtain a composite magnetic material Fe3O4/SiO2/TiO2.H2O with the surface TiO2 coating amount of 50-55 wt %, wherein the specific saturation magnetization is more than 40 emu/g.
d) Under the same operation conditions, slowly drop the solution A2 into the reaction system, control the amount of zirconium isopropoxide added into per gram of Fe3O4/SiO2 at 1.5-2.5 mL, and perform stirring reaction at room temperature for 4-6 h after dropping. Separate an obtained precipitate by using an external magnetic field, wash with anhydrous ethanol, and dry in a vacuum oven at 60-80° C. for 10-12 h to obtain a composite magnetic material Fe3O4/SiO2/ZrO2.H2O with the surface ZrO2 coating amount of 40-50 wt %, wherein the specific saturation magnetization is more than 40 emu/g.
2) Prepare a hydrated copper oxide, zinc oxide, nickel oxide and cobalt oxide transition layer by adopting a surface deposition precipitation method
a) Dissolve soluble sulfates, acetates, nitrates or chlorides of Co2+, Ni2+, Cu2+ and Zn2+ in 100 mL of anhydrous ethanol or isopropanol to form solutions C—Co, C—Ni, C—Cu and C—Zn, wherein the molar concentration of ions in each solution is controlled within the range of 0.04-0.06 mol/L.
b) Add the composite magnetic carrier Fe3O4SiO2 coated with the dense single layer SiO2 on the surface into the prepared solution, and perform ultrasonic dispersion for 30 min, wherein the concentration by mass-to-volume ratio of the Fe3O4SiO2 is controlled within the range of 0.005-0.015 g/mL.
c) Slowly drop a 0.02-0.05 mol/L NaOH water solution into the reaction system at room temperature, uniformly stir by using the polytetrafluoroethylene stirrer, and control the endpoint pHs of the ions as follows respectively: Zn: 6.5-8; Cu: 7-9 and NiCo: 10-12. Age the reaction system for 2-4 h at room temperature, then separate by using the external magnetic field, firstly wash with ultra-pure water till the pH is neutral, further wash with anhydrous ethanol, and dry in the vacuum oven at 60-80° C. for 10-12 h to obtain Fe3O4SiO2MO.H2O (M=Co, Ni, Cu or Zn) with the surface coating amount of 40-50 wt %, wherein the specific saturation magnetization is more than 40 emu/g.
2. Prepare a ferrocyanide adsorbent
Soak the prepared Fe3O4/SiO2/MOx.H2O (M═Ti, Zr, Zn, Cu, Ni and Co) in a hydrochloric acid solution of potassium ferrocyanide, wherein the concentration of the potassium ferrocyanide is 0.5-1.5 mol/L, the concentration of the hydrochloric acid is 1.0-2.0 mol/L, and the concentration by mass-to-volume ratio of the Fe3O4/SiO2/MOx.H2O is 0.01-0.03 g/mL. React the system at room temperature, and stir for 30 min every 2-4 h. Separate the precipitate by using the external magnetic field after reacting for 20-24 h, fully wash the precipitate with ultra-pure water till flushing liquid is colorless, further wash with anhydrous ethanol, and dry the sample in the vacuum oven at 60-80° C. for 10-12 h to obtain the black and blue magnetic core coated composite adsorbent.
3. The composite carrier Fe3O4/SiO2 adopted in the step 1 has the following characteristics:
The particle size of the magnetic core Fe3O4 is 10-60 nm, the specific saturation magnetization is more than 75 emu/g, and the content of organic matters is lower than 1%. The specific saturation magnetization of the composite carrier Fe3O4/SiO2 is more than 70 emu/g, and the oxidation resistance and the acid and alkali resistance are good.
4. The Fe3O4/SiO2K4−yMx[Fe(CN)6] composite adsorbent prepared in the step 2 has the following characteristics:
The particle size range of the sample is 0.2-5 μm, the specific saturation magnetization is 3-10 emu/g, and when the initial concentration of Cs+ in radioactive wastewater is 1-10 mg/L and competing ions H+, Na+ and K+ (the concentration is 0.1-1.0 mol/L) exist, the adsorption distribution coefficient Kd of the composite adsorbent for Cs+ is 104-107 mL/g, and the adsorption selectivity coefficients for Cs+ are as follows respectively: KsCs/H=103-106, KsCs/Na=103-105 and KsCs/K=103-104.
5. The stirrer adopted in all the steps is made of polytetrafluoroethylene, thereby being capable of preventing magnetic substances from adhering to the surface of the stirrer and preventing the phenomena of non-uniform dispersion of particles and non-uniform growth of a coating layer.
The invention has the following beneficial effects:
The magnetic core coated ferrocyanide composite adsorbent researched by the invention has the following structural characteristics: 1. the adsorbent takes Fe3O4 nanoparticles as the core, in order to ensure the magnetic separation effect of the composite adsorbent during the use, it is required that the specific saturation magnetization of Fe3O4 is more than 75 emu/g; 2. the SiO2 dense layer is coated on the surface of Fe3O4, so that the adsorbent can achieve the effects of inhibiting oxidation of the magnetic core material, stabilizing the magnetism of the material and improving the acid and alkali resistance of the material; 3. the hydrated oxide single layer of Ti, Co, Cu, Ni, Zn or Zr is coated on the surface of Fe3O4/SiO2, and such hydrated oxide can perform hydroxyl polymerization reaction with the surface of SiO2 to produce M—O—Si bonds, so that the metal ions can firmly grow on the surface of SiO2; and 4. the composite magnetic material Fe3O4/SiO2/MOx.H2O reacts with the potassium ferrocyanide solution to form Fe3O4/SiO2/K4−yMx[Fe(CN)6]. The metal ions M stabilize the ferrocyanide, achieve a bridge effect for bonding the ferrocyanide and the composite carrier together and further improve the bonding strength of the composite adsorbent. In addition, the ferrocyanide is in the outermost layer of the composite adsorbent, thereby being conductive to improving the effective utilization rate of the adsorbent.
The schematic diagram of the structure of the magnetic core coated adsorbent is as shown in FIG. 1. TEM, SEM and VSM determination results show that the particle size of the composite adsorbent is 0.2-5.0 μm, the specific saturation magnetization is 3-10 emu/g, and the magnetic separation effect in the external magnetic field is good. The determination of the adsorption performance for Cs+ shows that the adsorption speed of the adsorbent for Cs+ is fast, and when the initial concentration of Cs+ is 1-10 mg/L and the competing ions H+, Na30  and K+ (the concentration is 0.1-1.0 mol/L) exist, the adsorption distribution coefficient Kd of the composite adsorbent for Cs+ is 104-107 mL/g, and the adsorption selectivity coefficients for Cs+ are as follows respectively: KsCs/H=103-106, KsCs/Na=103-105 and KsCs/K=103-104. In addition, in the whole preparation process, no organic template is adopted, the composite adsorbent has no residues of organic matters, and the stability of the material in the use process is improved.